Networks of the aforesaid type are well known, their spanning devices serving to isolate local traffic (that is, traffic sourcing from, and destined for, stations on the same sub-network) to the sub-network concerned, at least to some degree. Examples of such networks are:
(a) bridged Ethernets where stations, identified by Ethernet addresses, are connected to sub-networks (also termed logical segments) which are interconnected by spanning devices in the form of bridges operating at level 2 of the seven-layer OSI Reference Model; and PA1 (b) Internet networks where stations identified by "ip" addresses are connected to "ip" sub-networks which are interconnected by spanning devices in the form of routers or gateways operating at level 3 of the seven-layer OSI Reference Model.
Knowledge of topological features of such networks (for example, the connection arrangement of stations to sub-networks and the interconnection of sub-networks by spanning devices) is of importance in monitoring and optimizing the performance of the network and in planning for its expansion to cope with increased demand. However, such knowledge is frequently difficult to ascertain, particularly with large networks. For example, keeping up-to-date plans of the network by recording every change made, is not only time-consuming but is, in reality, a virtually impossible task to carry through with complete accuracy; in addition, such plans can only indicate the intended connection state of the network and cannot take account of the failure of network elements such as the simple disconnection of a station from its sub-network. Physical inspection of a network to ascertain its topology is also very difficult since much of the network may be hidden beneath flooring or located at remote sites.
In the case of a network made up of a single sub-network, it is possible to ascertain which stations are connected to the subnetwork simply by monitoring the traffic on the sub-network; all stations sourcing traffic are obviously connected to the sub-network as there is no other source of traffic outside the sub-network. However, where the network comprises several sub-networks interconnected by spanning devices, it is no longer possible to make such simple deductions as traffic appearing on any one particular sub-network might have originated on another sub-network.